The present invention relates to a control device for a loading and unloading mechanism, and more particularly to a lift cylinder control applied to a fork lift truck. Specifically, the present invention is concerned with a control device for a loading and unloading mechanism wherein the system control for governing the operation of a lift cylinder is supervised by a microcomputer.
As is well known, a fork lift truck comprises a loading and unloading mechanism and a vehicle body. The loading and unloading mechanism comprises a vertically elongated guide rail called an "upright", and a fork slidable in the upright. The mechanism further comprises a hydraulic member, as for example, hydraulic cylinder for lifting and lowering the fork and tilting the upright.
In connection with the prior art loading and unloading control, for instance, lifting height control, drawbacks are pointed out as follows: Recently, there is a tendency that the lifting height becomes high when loading and unloading work is effected with a fork lift truck. For instance, the piling and unloading may be effected at heights greater than 10 m. In such a case, it is difficult for an operator to adjust the loading and unloading mechanism so that the fork is placed at the predetermined height, looking at the top of the fork positioned above about 10 m relatively to the seat of the operator. Accordingly, it is desirable for the operator to easily effect piling and unloading the load at the predetermined position.
In order to embody this requirement in the prior art, the upright is provided with a limit switch for stopping the fork at a predetermined position. When the fork reaches the predetermined position, for instance, 8.5 m, the control device is designed so as to light a lamp provided at the operator's unit or break a driving power supply for loading and unloading operations. Usually, a load is unloaded on a shelf with a plurality of steps. For this reason, in order to determine the desired position it is required to select the step. The provision of a predetermined number of limit switches, for instance ten, is required in order to meet the height of the shelf. Further, it happens that the piling and unloading is required at another shelf according to the change of the working place. In such a case, if the height of the shelf is different from that of the prior one, a more complicated control device is required. Actually, it has been impossible to effect the piling and unloading operation. Further, from the point of view of the system control in the prior art, a plurality of analog control circuits, such as, comprising combination of relay circuits respectively provided with respect to the controlled system, as for example, lifting height control are incorporated in the control unit of the control device for loading and unloading mechanism. Prior to the lifting work, an operator effects various settings according to the lifting height condition required for loading and unloading operation and then starts a lifting height work. In this instance, an automatic control system is constituted, which includes therein a valve opening control system provided with respect to a hydraulic pressure circuit for actuating a lift cylinder. The lifting height control is effected so as to control the valve opening control system due to the deviation between an actual lifting height above said setting value. However, when the setting is changed to a great extent according to the change of the loading and unloading working place, it is required to adjust the automatic control system in order to stabilize the control system. Alternately, it happens that the desired control accuracy cannot be obtained. Further, such a lifting height control is effected in a series of sequential control for loading and unloading work with the lifting height control being related to various kinds of controls. Accordingly, it is desirable to supervise the whole system control in view of the simplicity of the circuit and harmonious execution of the control.
In view of this, another attempt has been made. The programmed series of sequential control matching with the objective loading and unloading operation is stored in a computer, such as a microcomputer. When, for instance, lifting height control is effected, the concerned programmed routine for lifting height control is called from the program to effect a lifting height control due to the execution of the programmed routine.
In the prior art, when a load is piled or unloaded on a shelf with a fork lift truck into which a computer controlled device is incorporated, drawbacks are pointed as follows: The method of piling a load on a shelf comprises the steps of moving an unloaded fork lift truck near the shelf, lifting the unloaded fork to the predetermined shelf, running the fork lift truck at that position in the forward direction, mounting the load on the fork, lifting the fork within the same shelf by the predetermined height, running the fork lift truck at that position in the backward direction, and lowering the fork to the predetermined running position. On the other hand, the method of unloading a load from a shelf comprises the steps of moving the loaded fork lift truck near the shelf, lifting the fork to the predetermined shelf, running the fork lift truck at that position in the forward direction, lowering the loaded fork within the same shelf by the predetermined height, running the fork lift truck at that position in the backward direction, mounting the load on the shelf, and lowering the fork to the predetermined running position.
When the piling and unloading is effected with the above-mentioned fork lift truck, the desired shelf is selected by actuating an address selecting key switch corresponding to each shelf position provided on an operating box incorporated in the truck body. In this instance, the designation or selection of two height positions within the same shelf is required. Accordingly, the actuation of the address selection key switch is troublesome.
Further, an automatic loading and unloading control, such as, lifting height control is effected in accordance with the sequential control matching with the objective loading and unloading work. An operator actuates the address selecting key switch in order to designate the address allotted to the lifting height value. In this instance, if the address to which a desired lifting height data is assigned is erroneously designated, the automatic control is effected under the condition that the data stored in this address is selected as a lifting height objective value. This brings about a serious safety problem.
In an automatic loading and unloading control supervised by a microcomputer, in addition to the drawbacks stated above, when a lifting height control is actually effected in accordance with the sequential data in connection with a loading and unloading operation stored in the microcomputer, the following drawbacks are further pointed out: When the lifting speed of the fork is controlled by an automatic lifting height control effected due to the stored lifting height, if a command for changing speed is given, it has been difficult to effect a follow-up control because of the fact that the characteristic of the opening angle of the lift valve with respect to the lifting or lowering speed of the fork in non-linear, and that there exists a response delay inherent in the automatic control system. Furthermore, when the fork reaches the objective lifting height and then is stopped thereat, there is not provided a mechanism for slowly stopping the fork. Accordingly, the fork is stopped suddenly, which brings about a safety problem.